1. Field of the Invention
The invention relates to a discharge lamp, especially a discharge lamp using a cathode into which a sintered body of a powdered metal with a high melting point and of a powdered emitter material is provided. The invention, furthermore, relates to a mercury lamp.
2. Background of the Disclosure
Discharge lamps of the short arc type, such as a xenon lamp and a mercury lamp are generally widely used as a light source for purposes of illumination, as a light source for a projector and as a light source of a semiconductor exposure device.
In these discharge lamps, a cathode and an anode are arranged opposite one another, a few millimeters apart in a fluorescent tube. For the cathode, a so-called thoriated tungsten is used in which the tungsten contains thorium oxide, and a tip of the cathode is cut in a conical shape for producing stable arc discharge.
However, during luminous operation of the lamp, the cathode has a high temperature due to arc discharge. The above described conical shape is, therefore, deformed by vaporization or melting of the cathode material. This change of the cathode shape causes fluctuation of the position of an arc bright spot which is formed between the electrodes. Consequently, in a device with an optical construction based on the position of the arc bright spot, the desired amount of light can no longer be obtained. Furthermore, the vaporized cathode material adheres to the inner side of the fluorescent tube as a blackened substance; this reduces the amount of light from the lamp. The reduction of the amount of light has major disadvantages, for example, in a lamp which is used for a projector.
On the other hand, a process is also proposed in which a sintered body with a good electron emission characteristic is inserted on the tip of the electrode of thoriated tungsten, and in which electron emission is caused at a low temperature and thus luminous operation of the lamp is accomplished. In the cathode in which this sintered body is fitted, however, it was regarded as disadvantageous that the process of inserting the sintered body fitting is not always possible without problems due to difficulties in cutting to size, and that, in the course of luminous operation of the lamp, the sintered body falls out or changes its position.